Previous attempts for adding an automatic gain control (AGC) pulse or back porch pulse in a horizontal blanking interval (e.g., a portion of the back porch) has caused various degrees of playability artifacts.
For example, the copy protection system disclosed in U.S. Pat. No. 4,163,253 to Morio, adds back porch AGC pulses in a consecutive manner during the active video field. This causes a noticeable darkening effect on many television sets that utilize back porch sample clamping circuits. For example, see the AGC pulses 12 shown in dotted lines in FIG. 1.
A variation of the Morio patent adds the back porch AGC pulses in a non-consecutive manner or in clusters (see U.S. Pat. No. 4,819,098 to Ryan). Adding the back porch signals in one out of every N television lines in the active television field, provides an improvement in reducing the darkening effect experienced in the system of the Morio ('253) patent. However, in some television sets the one out of every N lines system disclosed in the Ryan '098 patent causes a trace of darkening to be displayed on the one out of N lines, which causes a “venetian blind” (darkening) effect.
Similarly, adding AGC pulses in a cluster of television lines also decreases the darkening effect experienced by the system disclosed in the Morio ('253) patent. To this end, the clusters of the AGC pulses should be located away from television lines in the active field portion or viewable area of the video field. This restricts the number of clusters of AGC pulses which can be used.
Accordingly, adding back porch pulses in a 1/N manner seems to be successful in reducing the overall darkening effect of the Morio ('253) patent apparatus, but can lead to a venetian blind effect. This venetian blind effect is a darkening of every N line. For example, if the back porch pulses are to be added to one out of every ten lines, then for some displays every tenth line will exhibit a slight amount of darkening. This venetian blind effect may be more noticeable in newer television sets that have analog to digital converters and or digital processing.
A further alternative to the Morio ('253) and Ryan ('098) patents, is a system which adds a lowered back porch signal to a portion of the back porch of selected television lines to partially offset or to substantially cancel darkening effects caused when AGC pulses are added to the back porches. (For example, see U.S. Pat. No. 6,188,832 to Ryan.) However, the lowered back porch signal may cause some television displays to show an overly bright picture. So this solution can lead to a “brightening” artifact in some television sets.
Another copy protection system is disclosed in World Patent WO 2004/004346 A1 to Tan which shows the addition of an AGC pulse in each line of the video field throughout the visible or active television lines. These AGC pulses are added preferably prior to a color burst signal to avoid the darkening effects of the Morio apparatus. However, although the Tan process causes less darkening than the Morio process, noticeable darkening results in some television displays as well. See the AGC pulses of FIG. 2A, which also illustrates “narrowed” sync pulses 16 with the AGC pulses 12 following immediately from the trailing edge of the narrowed horizontal sync pulses.